The present invention relates to drivers, and, more particularly, to a dual mode IEEE 1394-1995/1394b compliant output driver circuit having an internal and external voltage bias.
Under Institute of Electrical and Electronic Engineers"" (IEEE) standard 1394, a serial bus architecture is capable of providing transmission of multimedia data. This IEEE 1394-1995 standard has revolutionized the transport of digital data for professional and consumer electronics products and computers. By providing an affordable high-speed method of interconnecting digital devices, a more versatile I/O connection has been established. This serial bus architecture is the basis for the integration of all devices involving the transport of digital data including electronic entertainment, communication, and computing, as well as, test and measurement, command and control, and computer peripherals. The scalable architecture and flexible peer-to-peer topology makes IEEE 1394-1995 ideal for connecting devices ranging from printers and computer hard drives to digital audio and video hardware with real time processing requirements for on-time multimedia.
An updated version of the IEEE 1394-1995 standard, IEEE 1394b, provides a high performance serial bus architecture for mass storage, consumer electronics and automotive applications. Specifically, the new standard IEEE 1394b not only doubles the speed of the previous generation of IEEE 1394-1995 devices up to 800, 1600 and 3200 megabits-per-second (Mbps), it also increases distances up to 100 meters. Thus, this updated standard opens the way for new applications for the 1394 high performance serial bus including the FireWire(copyright) serial bus architecture developed by Apple(copyright) and Texas Instruments Inc(copyright). These enhanced features allow designers of PCs, mass storage, consumer electronics and automotive applications to increase the functionality of designs while simplifying architecture and reducing costs. In particular, the IEEE 1394b serial bus design is ideal for high-bandwidth applications like multimedia. In addition to providing high-speed access, IEEE 1394b supports plug-and-play connections, hot swapping, multiple speeds on the same bus, and isochronous data transfer, as well as providing power to peripheral devices. Thus, the benefits of a system compliant with the new standard include a simple to operate, seamlessly integrated network of digital multimedia peripherals.
The IEEE 1394 standard defines a protocol including a serial bus management block coupled to a transaction layer, a link layer and a physical layer. The physical layer provides the electrical and mechanical connection between a device or application and the IEEE 1394 cable. The physical layer also provides arbitration to ensure that all devices coupled to the IEEE 1394 bus have access to the bus as well as actual data transmission and reception.
Architecture compliant with IEEE 1394 includes a central processing unit coupled to local memory and a link layer by a processor bus. The link layer couples to a physical layer using control and data signals. The physical layer connects directly to the cable that couples all other peripheral devices in a network or system.
Difficulties arise when a peripheral having the old 1394-1995 standard couples to a network that is compliant with the new standard. Specifically, the driver in the physical layer of IEEE""s 1394-1995 standard is regulated over a range of external biasing voltages. In contrast, however, a driver in a physical layer in compliance with the IEEE 1394b standard is regulated over a range of internal biasing voltages.
There is a need for communication between devices that use both IEEE 1394-1995 and IEEE 1394b as their basic transport layer of the network communications system. Accordingly, some drivers compliant with IEEE 1394b must be backwards compatible with any peripheral device that complies with the old standard, IEEE 1394-1995.
Thus, there exists a need for a dual mode driver, having the capability of being externally biased in compliance with IEEE 1394-1995 standard and the capability of being internally biased in compliance with IEEE""s 1394b standard.
To address the above-discussed deficiencies of driver incompatibility between the first standard, IEEE 1394-1995, and the latest standard, IEEE 1394b, the present invention teaches a dual mode output driver circuit within the architecture of a IEEE 1394b compliant physical layer (PHY) circuit. The output driver circuit in accordance with the present invention of a serial bus structure directly coupled to a cable in a system for digital data transfer to and from the cable over the bus structure includes a current source, a first sub-circuit portion, a second sub-circuit portion, a switch and an amplifier. The first sub-circuit portion includes a reference voltage node. The current source connects to both the first and second sub-circuit portions to provide current. The second sub-circuit portion includes an external voltage bias node and a common mode voltage node, where the external voltage bias node connects to the cable. The switch couples between the first and second sub-circuit portions to provide an internal voltage bias operation mode and an external voltage bias operation mode for the output driver circuit in response to an enable signal. The switch couples to receive a reference voltage through the reference voltage node and a common mode voltage through the common-mode voltage node. In addition, an internal bias voltage is supplied to the switch. In the internal voltage bias operation mode, the amplifier connects the switch to the first and second sub-circuit portions to amplify and supply the difference between the common mode voltage and the internal bias voltage. In the alternative, the amplifier connects the switch to the first and second sub-circuit portions to amplify and supply the difference between the reference voltage and the common mode voltage while operating in the external voltage bias mode.
Thereby, the dual mode driver in accordance with the invention allows either an external voltage bias to drive current that is regulated over a range of external biasing voltages or an internal voltage bias to drive current within the driver. The output driver circuit forces the bias on the cable. The voltage bias is strong enough to overcome weak biasing from a receiver connected at the other end of the cable. In addition, proper drive currents are maintained through voltage matching of the reference voltage in the external bias mode.
Advantages of this design include but are not limited to a dual-mode output driver circuit having a significant decrease in silicon area and a significant decrease in output capacitance due to less required circuitry at the output of the driver. This output driver has enhanced matching due to minimal crowding of circuitry near the bondpad and due to a lower required power. In addition, the output driver in accordance with the present invention eliminates the need for redundant current voltage references required by an approach that makes use of multiple drivers to provide an internal bias voltage mode and an external bias voltage mode.